fix: escape blasttab plus in schema regex

[sapunyangkut]

Summary

• escape the literal + in the documented blasttab+ export format
• stop the schema regex from rejecting blasttab+ and accepting invalid values such as blasttabb

Closes #136.

Validation

• python -m json.tool nextflow_schema.json
• Python and JavaScript regex boundary checks for valid, invalid, and comma-separated values
• git diff --check

PR checklist

• This description explains the bug and the one-token schema correction.
• The change is limited to the existing schema regex; pipeline runtime tests and test-data changes are not applicable.

[charles-plessy]

Thanks a lot ! Please note that proper blasttab+ support will need to upgrade LAST to version 1652 as the older versions do not output the extra columns when converting MAF files produced by last-split.